The game is set to look for the name of the category, only. If you want to add pictures, you find the specific girl's folder under Resources --> Characters, drop the pictures in, and make sure all the files are appropriately named (Anal__, Les__, Sex__, where __ is any previously unused number.) If you're looking to add pictures that will apply to *all* girls, there's also a "Default" folder under Characters that the game will pull from if there's nothing in a category for a specific girl, so you could add generic pictures that fit a category there.